In the development of motor vehicles having lower fuel consumption, an increasing amount of attention is paid to the portion of fuel consumption caused by the electrical components in motor vehicles. It is assumed that the additional fuel consumed to generate 100 W of electrical power is in the magnitude of 0.1 to 0.15 l of fuel per 100 km driving distance. It follows that electrical machines used in the motor vehicle, e.g., a generator, are required to output electrical power with optimal efficiency.
Today, the generation of electrical power in motor vehicles usually takes place using claw-pole alternators. These alternating-current machines are connected to the vehicle electrical systemxe2x80x94which is a direct-voltage networkxe2x80x94via a passive diode rectifier bridge. The generators used to produce electrical energy in motor vehicles are dimensioned so that they can deliver the electrical power required to supply the electrical components even when the engine is idling.
In order to meet the expected increase in demand for making electrical power available in motor vehicles in the future as well, alternators, such as a claw-pole alternator, can be outfitted with pulse-width modulation inverters. Using these components, the output of the alternator can be increased considerably, especially in its lower speed range.
Today, alternators are usually dimensioned in such a fashion that, together with the diode rectifiers, they already start outputting electrical power below the idle speed of an internal combustion engine. The speed at which the generator reaches a terminal voltage of 14 V with full excitation is in the magnitude of 1000 to 1200 minxe2x88x921. During idle (at a generator speed of approximately 1800 minxe2x88x921), the power output reaches a tangent point of the power curve. At this operation point of the alternator, the output of electrical power reaches approximately 60 to 70% of its maximum value. The tangent point mentioned is usually set in the engine idle of the internal combustion engine. The electrical machine is most efficient at the tangent point.
The maximum value of the output of electrical power that occurs is reached at a generator speed of approximately 6000 minxe2x88x921 and increases practically no further at higher speeds.
Although this dimensioning of an electrical machine working in the generator mode results in a very good compromise between size and the requirement that electrical power be completely covered, even when the internal combustion engine of a motor vehicle is idling, the electrical machine operates within a very high speed range close to its short-circuiting point to output maximum electrical power. As a result, the stator copper losses that occur are considerable.
The operation of an electrical generator having allocated pulse-width modulation inverter allows the number of stator windings on the stator of the generator to be designed according to design criteria other than the inception speed of the generator at which the output of electrical power sets in. By means of the pulse-width modulation inverter allocated to the electrical machine, the power output of the electrical machine can be directed along the torque line in the lower speed range, independently of the number of coils in the stator winding of the generator. By means of the operating mode of the electrical machine with downstream-installed pulse-width modulation inverter, the inception speed at which the electrical machine outputs electrical power can be reduced so far that sufficient electrical power can be output even when an internal combustion engine is idling. By operating the electrical machine using pulse-width modulation inverters, therefore, sufficient electrical power is available, even at low internal combustion engine speeds. It is no longer necessary to rely on the terminal voltage of the machine exceeding the nominal voltage.
If the pulse-width modulation inverter is operated using an electrical machine, the stator winding of which comprises a smaller number of coils, the following advantages can be attained in the upper speed range:
On the one hand, in the upper speed range, either the output of the electrical machine can be increased, or its efficiency can be significantly enhanced. In electrical machines having a small number of coils, compared to electrical machines having a larger number of coilsxe2x80x94with electrical output the samexe2x80x94a lower slot electrical loading is attained with the electrical machine having a smaller number of stator windings. Assuming a slot space factor that is equal for the electrical machine having a larger number of coils and for the electrical machine having a smaller number of coils, markedly fewer losses in the winding copper occur in the electrical machines having a smaller number of coils. Moreover, the losses occurring in the iron caused by the slot electrical loading are also reduced (harmonic losses). The machine winding of the electrical machine is therefore able to be dimensioned for minimal total losses in one driving schedule.
Using the means proposed according to the invention for attaining the object of the invention, i.e., to operate an electrical machine having a small number of coils using a pulse-width modulation inverter, the performance of these electrical machines can be fully utilized in the upper speed range. As a result, there is a power reserve available in terms of making electrical power available.